Microwave oven



Feb. 25,l 1969 R. L. COUGOULE MICROWAVE OVEN Filed OCT.. 14, 1966 UnitedStates Patent 3,430,022 MICROWAVE OVEN Robert L. Cougoule, San Carlos,Calif., assignor to Litton Precision Products, Inc., Sau Carlos, Calif.,a corporation of Delaware Filed Oct. 14, 1966, Ser. No. 586,684

U.S. Cl. 219-10.55 2 Claims Int. Cl. H05b 9/06 ABSTRACT OF THEDISCLOSURE A microwave oven is provided which includes a source ofmicrowaves coupled to the microwave heating cavity by means of awaveguide. The waveguide is short circuited at a position remote fromthe microwave source and includes a pair of microwave passages or feedmeans for passing microwave energy from the waveguide into the heatingcavity. The first microwave passage means contains a pair of parallelspaced narrow slots elongated in the width direction of the waveguidefor favoring the introduction of E-field from said waveguide into thecavity. The second microwave passage means is located between the firstmicrowave passage means and the short circuited position of thewaveguide and contains a large iris or opening, substantially larger inarea than either of said narrow slots, for favoring the introduction ofH-field from the waveguide into the cavity.

The present invention relates to microwave ovens and, more particularly,to an improved microwave oven which provides uniform heating patterns inthe oven without requiring any moving or rotating mode mixers.

In known microwave cooking or heating devices, standing waves of highfrequency are set up within the oven cavity due, for example, toreflection of the supplied high frequency waves from the cavity walls.Such standing wave comprises locations of high electric fieldintensities and low electric field intensities at different locationswithin the oven cavity. This results in relatively hot or cold spots atvarious locations therein and, with a large piece of food or withseveral pieces placed about the cavity, causes some portions of food tobe heated more than other portions.

In order to reduce these hot and cold spots, it has generally been thepractice to increase the number of modes excited within the cavity bythe addition of either fixed or moving elements. This is possible sincea cavity having its length, width and height different in dimension andlarge relative to the Wave length of the microwave source is inherentlycapable of sustaining a plurality of different modes. By adding togetherthe electric fields of many different modes, a substantially moreuniform electric field is obtained than is obtainable with a singledominant lInode and the magnitude of the differences between the highand low electric field intensities at different locations within thecavity is considerably reduced.

The usual solution for solving this problem is the mode stirrer, afan-like unit consisting of many blades each of which refiects incidentmicrowave enenrgy within the cavity and which is rotated therein by amotor. By rotating the stirrer within the cavity, the aforesaid high andlow field intensity spots, or hot and cold spots, are sequentiallyshifted to different locations around the oven and each portion of alarge piece of food on the average over a period of time receivesapproximately the same amount of heating energy as it is sequentiallyexposed to high and low intensity fields.

Although this method of distributing the heating field within the cavityhas proven satisfactory, it possesses the obvious disadvantages ofrequiring additional moving parts. From the point of View of themanufacturer, this is undesirable since such parts wear and requireadditional expense.

Various proposals have been made in the prior art for exciting aplurality of modes without any moving parts. One such proposal isdisclosed in U.S. Patent No. 2,943,- to G. Guanella. Numerous othersappear in the prior art, .but none have proved satisfactory.

Therefore, it is an object of the present invention to provide a novelmeans for uniformly heating food within a microwave oven without the useof moving elements.

It is an object of the present invention to distribute simultaneously a.substantially uniform microwave heating field to substantially alllocations within the cooking chamber.

It is a further object of the present invention to distribute the energyto various locations within the ycooking chamber by a purely static orfixed device which is less expensive to manufacture and which requiresno maintenance.

It is a further object of the present invention to provide means forsimultaneously exciting as large a number of the modes of high frequencywaves as is possible within the given cavity, the cooking chamber, theaddition of which avoids hot or cold spots.

In accordance with the present invention, a fixed nonabsorbing,relatively lossless substantially fiat conductive surface or plate isprovided and is located within the cavity exposed directly to the food.It is mounted by an insulator `means so that it substantially obstructsa direct path between the food and location at which microwave energy isintroduced into the cooking chamber. The plate is positioned with itssurface substantially parallel to the food supporting tray. Moreover, ina preferred embodiment, distributed feed passages or irises of differentkinds are utilized to introduce microwave energy into the cavity inorder to enhance the uniform distribution of the electric field withinthe oven cavity. One passage may be a susceptively stubbed iris whileanother may be a simple resonant iris.

The -foregoing and other advantages and objects of the present inventionwill become apparent from a consideration of the following detaileddescription and drawings of which:

FIGURE 1 is a side view of a microwave oven embodying the invention inwhich the side wall has been removed for purposes of illustration.

FIGURE 2 is a cross section of the bottom view taken along line 2-2 ofFIGURE 1.

FIGURE 3 is a cross section similar to the cross section of FIGURE 2 butembodying other forms of the invention.

FIGURE 1 shows an oven cavity 1 having a front door Wall 2, a rear wall3, a bottom wall 4, side wall 5 and top Wall 6. The oven cavity alsocontains a side wall 7 which, in FIGURE 1, has been removed for purposesof illustration. Within the cavity, there is shown a dielectric tray 8,lpreferably of relatively lossless dielectric material but which maytake the form of a metal rack, for receiving food. Tray 8 is supportedparallel to the top wall 6 by supports 9. A well-known magnetronoscillator or other suitable source of microwave energy is symbolicallysho'wn by the block labeled 10.

In the usual manner, the walls of the cavity are constructed of materialimpervious to microwave energy so as to prevent leakage from the cavity.The walls are sealed together to further prevent leakages through theboundaries. Moreover, the door wall of the cavity, through which food isintroduced into the cavity, contains the usual high frequency seals andelectrical interlocks which have been omitted for clarity.

In the preferred form of the invention, the material used for the wallsis stainless steel. Not only is stainless steel impervious to microwaveenergy, but it is relatively lossy or absorptive relative to manv othermetals or metallic compositions. Being lossy, a certain electral load ismaintained on the magnetron oscillator by the cavity during periods ofoperation. In so doing, the magnetron is protected to an extent againstno-load operation which might otherwise destroy the magnetron.'Microwave energy is fed from the magnetron oscillator through acoupling means such as the waveguide 11 although other forms of couplingcan be utilized.

In the preferred embodiment, microwave energy is transferred fromwaveguide 11 into cavity 1 through a unique distributed feed arrangementconsisting of a susceptively stubbed iris in the form of two traverseslots 12 and a resonant iris in the form of a rectangular passage 13.This feed arrangement is more particularly shown in FIGURE 2, laterdiscussed. These passages or irises may be physically open or, as analternative, may be closed with any material known in the art that ispervious to microwave energy but which prevents vapors from entering thewaveguide. The waveguide 11 also has a closed end 14 which is spacedfrom the resonant iris 13 by approximately a quarter wave length, or amultiple thereof, at the microwave source frequency.

Further, in accordance with the present invention, a fiat nonabsorbingconducting plate 15 is provided, supported within the cavity, spacedfrom the top, bottom and side walls of the cavity and maintainedparallel to the top wall 6 and dielectric food tray 8 by an insulatingmeans shown as insulator 16 to which it is fastened by a screw 17. Theinsulator is shown supported by the top wall of vthe oven cavity. Theplate 15 substantially obstructs a dilrect :path from at least one of`t'he irises to the lower half of the cavity. Plate or surface 15 may beconstructed of cold rolled steel, aluminum or any other metal.

A cross section of the microwave oven taken along line 2--2 in FIGURE 1is shown in FIGU'RE 2. Sidewall 7, deleted from FIGURE l, has beenincluded in FIGURE 2. This view shows the front door wall 2, side walls5 and 7, rear wall 3 and top wall 6 previously discussed with respect toFIGURE l. FIGURE 2 discloses in greater detail the preferred form of thesubstantially lossless oonductive plate 15 and the distributed feedmeans utilized in accordance with the invention. The plate 15 issupported on insulator 16 and aiiixed to the insulator, for instance, bymeans of a screw 17. The plate utilized in this embodiment is generallyrectangular but has two opposed edges, 18 and 19, which have a curvatureextending in a concave manner toward the plate center. The two-remaining edges 20 and 21 are straight and parallel. The .plate in thisembodiment has its front edge 21 nearer the front door wall portion 2 ofthe cavity than its back edge 20 is to the rear wall 3 and exposes aportion of iris 12. This construction of the preferred embodiment hasbeen found by experiment to attain better results.

The edges 18, 19, 20 and 21 of the plate are spaced from the Walls ofthe cavity. 'Because of the complexity in mathematically determining thetype and locations of the modes within a cavity, results are generallyfound experimentally. The arrangement and construction of the platedisclosed in FIGURE 2 has provided a substantially uniform heatingfield. In accordance with the invention, other fiat shapes and positionsof the plate in a plane parallel to the food tray are possible; and theoptimum for each particular oven of a different size or shape andassociated feed means utilized may be found experimentally.

FIGURE 2 also discloses in greater detail the arrangement of the irisesor passages used to couple microwave energy from the waveguide 11 intothe cavity 1. In this embodiment, a distributed type of coupling or feedmeans has been utilized to introduce microwave energy into the cavity;first, for introducing equal amounts of power at each of two respectivelocations along the top wall 6 of the cavity and, second, for maximizingthe number of rmodes within the cavity, each of which enhances theuniformity of heating therein. Located in the top wall 6 of the cavity 1is a first pair of slots 12 which comprise a susceptively stubbed irisas is known in the art. Spaced therefrom is a rectangular passage 13which is known as a resonant iris. The waveguide 11 contains irisesidentical to those located directly overlaying those in the -top cavitywall 6 in order to form passages for introducing microwave energy fromthe waveguide into the cavity. However, instead of cutting irises in thetop wall of the cavity, a large opening in the top Wall might be cutaway to allow the waveguide to fit therein and have its irises andsurface exposed directly t-o the cavity. These irises may physically beOpen or contain a window of any well known material pervious tomicrowave energy while preventing the introduction of vapors from theoven into the waveguide. The waveguide also contains a closed end 14spaced approximately a quarter wave length, or a multiple thereof, fromthe resonant iris, rectangular opening 13, in order to provide a shortcircuited termination to the waveguide which maximizes the amount ofpower introduced 'into the cavity from the waveguide. It is believedthat a desirable feature is to have the resonant iris 13 proximate theclosed end of the waveguide rather than the iris 12.

The narrow slots 12 forming the susceptively stubbed iris, as is known,favors the passage of electric field or E waves while the resonant irispassage 13, favors the passage of magnetic field or H waves from thewaveguide into the cavity. The waveguide 11 coupling the high frequencyenergy from the oscillator 10 to the cavity 1 is designed to support thepropagation therebetween of what is known as a traverse electric wave,that is a propagating wave containing both electric fields E andmagnetic fields H in which the magnetic field H has its vector in thedirection of propagation and the electric field E has its vectorstraverse to the direction of such propagation. Any of the standard textson waveguides give a mathematical analysis of such phenomena.

Moreover, a cavity such as 1 is capable of sustaining different modes offield distribution therein depending upon whether an E field or an Hfield is coupled from the waveguide into the cavity. Thus, byintroducing an E field at one location and an H field at another, agreater number of modes are excited, enhancing the result desired of-maximizing the number of excited modes for obtaining uniformity ofheating, than is generally possible with two couplings each of whichcouples either E fields or H fields alone.

Further, as is known, a high frequency passage such as the disclosedirises possesses a definite value of resistance, termed radiationresistance, governed in part by the dimensions of the passage. Since itis desired to distribute the high frequency power evenly through thecavity, it is desirable that equal amounts of power enter the cavitythrough each of the t-wo irises 12 and 13. This would not be possible ifthe two irises 12 and 13 possessed equal values of radiation resistancebecause, in that instance, maximum power would be coupled throu-gh thefirst iris and less through the second iris. Hence, in the presentinvention, the irises are dimensioned so that these values of radiationresistance are unequal. In this light, it may be noted that resonantiris 13 while favoring the passage of the H field from the waveguideinto the cavity allo-ws, in addition, some E field to pass.

FIGUR-E 3 shows another form of the invention which -may be utilized inparticular ovens. This figure is a cross sectional view of an ovensimilar to the view ofthe preceding form of this invention shown inFIGURE 2. In FIG- URE 3, the conductive plate 22 shown is rectangular inshape. This rectangular plate 22 is mounted symmetrically with respectto the front wall 2 and rear wall 3 and also with respect to side walls5 and 7 of the cavity 1. A single iris is formed in the top wall 6 ofthe cavity shown as a rectangular opening 23 in top wall 6 in order tointroduce the microwave energy into the cavity. Overlying this is a likepassage formed in the waveguide 11, the latter being 4affixed by aiiange 24 to the top wall 6 of the cavity 1.

In this embodiment, the plate 23 is mounted, as an example, on twoinsulators 25 by screws 26. In this embodiment, the insulators 25 arealso shown as being attached to the top wall 6 of the cavity. However,as is obvious, the plate 22 may be supported from other walls or by alarger number of insulators while maintained parallel to the food tray.lIn accordance with the teachings of the invention, the plate 22substantially obstr-ucts the passage of microwave energy directly fromthe waveguide to the food tray (not shown) and is also mounted parallelto the food tray.

In each of the disclosed embodiments, it will be noted that the mountingof the plate in front of the feeding irises causes microwave energy tobe reected back to the magnetron, an unavoidable property resulting inan increased voltage standing wave ratio, VSWR. However, by properlyspacing the plate from the top wall, this property is used to advantage.The phase of the reflected power is aligned such that any change in theelectrical load within the oven or cavity, such as due to thedifferences in size and moisture content of heated objects, from heavyto noload, never reiiects a VSWR in a phase near or in the magnetronscritical region, which would ordinarily cause the tube to mode and burnout. This feature, therefore, additionally acts as a protective devicefor the magnetron.

The cavity modes which are excited in the manner of the disclosedinvention possess particular utility for heating food articles ofrelatively flat shape or having a large length relative to their height.Referring again tol FIG- U-RIES l and 2, by mounting the plate 15parallel to the food tray .8 as in the disclosed oven, uniformity ofheating has been peculiarly successful in the planes parallel to andproximate the plane containing the food tray. Since one of the majorfunctions of the microwave oven is to supplement the ordinary oven forquickly reheatin-g precooked foods, the results assume a greatersignificance. Mounting the iiat plate 15 parallel to the food tray 8maximizes the number of modes in the desired manner and more so in theproper direction although results otherwise seem mathematicallycompletely unpredictable and are mathematically too complex todetermine.

As is evident from the standard texts and as hereinbefore mentioned,various modes can exist within a cavity each of which is characterizedby three integral numbers corresponding to the length, width and heightdimensions of a cavity. These numbers indicate the number of half wavesof the standing wave pattern for the particular mode along each of thethree dimensions of the cavity. The lmounting of the conductive plate 15as disclosed and the resulting excitation of a uniform field especiallywithin those planes 4proximate to and parallel to the food tray #Sindicate that the proper mode excitation has been achieved as a majorfunction of the height dimension or distance of the plane containing thetray from the plane of the conductive plate.

The foregoing embodiments of this invention have been described forpurposes of illustration only and are not intended to limit theinvention as set forth within the spirit and scope of the appendedclaims.

What is claimed is:

1. A microwave oven comprising a microwave cavity bounded by conductivewalls, a source of microwave energy, waveguide means connected betweensaid source and one of the walls of said cavity for coupling microwaveenergy therebetween, said waveguide means having a short circuited endremote from said source, tray means within said cavity for supportingobjects to be heated by microwave energy, means supporting said traymeans parallel to said one of the walls of said cavity, a relativelylossless plate of conductive material having a surface exposed directlyto said tray means mounted on insulator means within the cavity spacedfrom the walls thereof and substantially parallel to said tray means,and feed means for coupling microwave energy from said waveguide meansthrough said one wall and into said cavity, said feed means comprising:iirst microwave passage means having a pair of spaced narrow slotselongated in the width direction of said waveguide, and parallel forfavoring the introduction of E-eld from said waveguide into said cavityand second microwave passage means located between said first microwavepassage and said short circuited waveguide and comprising a largeopening, substantially larger in area than either of said narrow slots,for favoring the introduction of H-iield from Said waveguide into saidcavity.

2. A microwave oven comprising a microwave cavity bounded by conductivewalls, a source of microwave energy, a/waveguide connected between onewall of said cavity and said source for coupling microwave energytherebetween, said waveguide having a short circuited end remote fromsaid source, and means for feeding microwave energy from said waveguideinto said cavity comprising: iirst micro-wave passage means havin-g apair of spaced narrow slots elongated in the width direction of saidwaveguide and parallel for favoring the introduction of E-iield fromsaid waveguide into said cavity, and second microwave passage meanscomprising a large opening, substantially larger in area than either ofsaid narrow slots, for favoring the introduction of H-eld from saidwaveguide into said cavity.

References `Cited UNITED STATES PATENTS 2,704,802 3/ 1955 Blass et alM9- 10.55 3,263,052 7/ 1966 Jeppson et al 219-10.55 3,300,615 1/1967Smith 2l9-10.55

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

